The influence of seed water content on the oxygen effect in irradiated barley seeds

Abstract

Himalaya barley seeds were brought to eleven different water contents, ranging from 2 per cent (very dry) to 14 per cent (moist), by storage at various constant humidities. The seeds were then placed in glass vials which were evacuated and sealed off while still under vacuum so that the seeds remained in an anoxic atmosphere at room temperature both during irradiation and during post radiation storage periods ranging from 2 min to 4 weeks. They were irradiated with a 5500 c Co 60 γ-ray facility and hydrated in oxygen-or nitrogen-saturated water. The doses varied depending on the water content of the seeds and the hydration atmosphere. The magnitude of the oxygen effect at different seed water contents was measured in terms of oxygen-effect factors. These factors are the ratios of radiation doses that are required to produce the same amount of biological effect in seeds hydrated in nitrogen-saturated water and in those hydrated in oxygen-saturated water but otherwise treated identically. Oxygen-effect factors, as measured by injury (reduction in M 1 seedling growth), increased from 1 to 9 as seed water content decreased from 12·7 to 2·5 per cent. In very dry seeds (2·1 per cent water content) oxygen-effect factors of 10 for injury, of 15 for chromosome aberrations, and of 10 or higher for mutation frequencies were obtained. The response to oxygen rehydration of irradiated seeds with a low (4 per cent and lower) or with a high water content (12 per cent and above) showed little or no change during a two-week storage period in sealed evacuated vials. Furthermore, water content did not modify the influence of storage if the irradiated seeds were rehydrated in nitrogen-bubbled water. However, seeds with a medium water content, particularly about 10 per cent, exhibited a marked oxygen response at the beginning of the storage period but a very reduced response after 2 or 4 weeks storage. This reduced oxygen response was measured in terms of injury, chromosome aberrations and mutations. In these seeds the electron spin resonance signal amplitude decreased significantly and in a fashion parallel to the decrease in oxygen sensitivity. These results reveal that some of the damage in irradiated seeds may be initiated by interactions between oxygen and radiation-induced radicals. The lifetime and behaviour of the radicals and the behaviour of oxygen depend on the seed water content.